"We're moving to 1TB because 'unlimited' wasn't a feature that we discussed very much or a feature that our users were that familiar with," a Yahoo spokeswoman explained in a statement to The Times. "We opted to move to 1TB instead, because we believe by having a huge limit like 1TB it will inspire our users to store more in Yahoo Mail."

The company said that no user of the free version of Yahoo Mail has ever filled up 1 terabyte of space. And if someone does, the company promises to make changes necessary to accommodate them. One terabyte is equal to 1,024 gigabytes.

"If people need more in the future, we are committed to staying ahead of need there," Jeff Bonforte, Yahoo senior vice president of communications products, said in a statement.

Why is Yahoo making this confusing change? Marketing.

The Sunnyvale, Calif., company wants to be able to boast about how much free storage space Yahoo Mail offers compared with other services, such as Google's Gmail, which offers 15 gigabytes of free storage.

The Yahoo Mail change comes as the company celebrates the 16th birthday of the email service. As part of that, Yahoo said it is also redesigning the Yahoo Mail website and rolling out updates for its apps. Some features that were previously only available to paying customers are now available to all, and users can now also choose themes for their inbox -- rather than just having a plain white background.

[For the record, 1:11 p.m. PDT Oct. 8: An earlier version of this post reported that Microsoft's Outlook.com email service has a user storage limit of 7 GB. Microsoft said storage is unlimited unless the account holder uses their account to send spam.]

SAN FRANCISCO -- Technology giants Facebook, Google and Yahoo presented a rare united front in filing separate motions asking to publicly disclose more details about secret national intelligence requests they receive under the Foreign Intelligence Surveillance Act.